In one embodiment, tappets are used as a mechanism for facilitating the lifting of valves in engines and pumping fuel in fuel pumps. An example of a prior art tappet and associated components is shown in FIG. 8. Referring to FIG. 8, a tappet 808 is shown having a bottom portion and cylindrical walls extending up from the bottom portion which, together, define an outer casing 809. Additionally, tappet 808 includes a roller 806. As oriented in FIG. 8, outer casing 809 reciprocates or oscillates vertically within an inner cylindrical bore 816 of a cylinder block 814. Tappet 808 is driven in an upward direction by the rotation of a cam 802 having an irregular shape as shown, such as a lobe shape. More specifically, the rotation of cam 802 (e.g., in a counterclockwise direction) drives roller 806 (which is connected to the remainder of tappet 808 via a connection mechanism 804) in an upward direction (as oriented in FIG. 8) as an apex 818 of cam 802 approaches roller 806. Roller 806 (and thus the entire tappet 808) will be at its highest or “top” position when apex 818 is in contact with roller 806. As cam 802 continues to turn so that apex 818 is no longer in contact with roller 806, a pushrod 810 (itself and/or a spring mechanism (not shown)) pushes tappet 808 in a downward direction, causing roller 806 to remain in contact with cam 802. The top portion of tappet 808 (not shown) may be connected to a mechanism facilitating the opening and closing of valves or pumping fuel, for example.
A problem that can occur when tappet 808 operates within cylinder block 814 is that, during operation, tappet 808 can begin to rotate about its central axis L. Such rotation can damage tappet 808 and associated components because those components may collide with each other during rotation and/or affect the connection of tappet 808 to a fuel pump, cylinder block, etc. While contact between cam 802 and roller 806 during oscillation is generally sufficient to prevent tappet 808 from rotating, rotation nonetheless sometimes occurs when tappet 808 is at its top position, and occasionally when it is at its bottom position as well. To prevent such rotation, mechanisms such as a guide screw 812 have conventionally been placed through cylinder block 814 to fit within a vertical groove or elongated slot 820 cut into the outer casing of the tappet 808.
For reasons including the desirability to reduce stress on various components by reducing the moving or rotating mass of tappet 808 at high speeds, it has become desirable to make outer casing 809 of tappet 808 increasingly thin. As a result, anti-rotation mechanisms such as guide screw 812 shown in FIG. 8 can become ineffective. For example, guide screw 812 can cause tappet 808 to bend and lose its roundness if outer casing 809 of tappet 808 is thinner than that shown in FIG. 8.